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SDSS J1021+1744 is a detached, eclipsing white dwarf / M dwarf binarydiscovered in the Sloan Digital Sky Survey. Outside the primary eclipse, thelight curves of such systems are usually smooth and characterised by low-levelvariations caused by tidal distortion and heating of the M star component.Early data on SDSS J1021+1744 obtained in June 2012 was unusual in showing adip in flux of uncertain origin shortly after the white dwarf's eclipse. Herewe present high-time resolution, multi-wavelength observations of 35 moreeclipses over 1.3 years, showing that the dip has a lifetime extending overmany orbits. Moreover the "dip" is in fact a series of dips that vary in depth,number and position, although they are always placed in the phase interval 1.06to 1.26 after the white dwarf's eclipse, near the L5 point in this system.Since SDSS J1021+1744 is a detached binary, it follows that the dips are causedby the transit of the white dwarf by material around the Lagrangian L5 point. Apossible interpretation is that they are the signatures of prominences, aphenomenon already known from H-alpha observations of rapidly rotating singlestars as well as binaries. What makes SDSS J1021+1744 peculiar is that thematerial is dense enough to block continuum light. The dips appear to havefinally faded out around 2015 May after the first detection by Parsons et al.in 2012, suggesting a lifetime of years.

A large, long-lived structure near the trojan L5 point in the post common-envelope binary SDSS J1021+1744